DESCRIPTION: Despite massive studies on protein biosynthesis, a complete picture of the actual events of this process still awaits molecular models. The universal cell organelles facilitating the translation of genetic information into proteins are ribosomes. Bacterial ribosomes are nucleoprotein assemblies, of over 2.3 million dalton, containing 57-73 different proteins and 3 RNA chains of about 4500 nucleotides, arranged in two subunits of unequal size. The long-term goal of this project is to illuminate the molecular mechanism and the dynamics of protein biosynthesis, by crystallography, supported by biochemistry, metalo-organo chemistry, genetics and electron microscopy. X-ray data collection with intense synchrotron radiation became feasible after the pioneering introduction of cryogenic techniques. The current objectives are to elucidate the structures of those ribosomal particles (the large subunits of ribosomes from Haloarcula marismortui) which diffract to the highest resolution (2.9 A), and of complexes, mimicking defined functional states, containing ribosomes and selected compounds participating in protein biosynthesis, such as mRNA, coding for synthetic sequences or naturally occurring proteins; charged cognate tRNA molecules and newly synthesized oligopeptides, of lengths controlled by omitting a selected amino acid from the reaction mixture. To facilitate these objectives a multi-directional phasing algorithm has been developed, which has already led to the construction of the first SIR MAP at low resolution. Several methods are being exploited: (a) MIR-MAD techniques: soaking in multi-metal salts, as well as direct binding of monofunctional reagents of undecagold and tetrairidium clusters to specific sites on the ribosome and on tRNA, in a fashion which does not hamper their functional activities; (b) non-MIR methods: real and reciprocal space searches, packing-motif identification, direct methods, maximum entropy and anomalous solvent contrast. The particle's envelop, constructed by a combination of the latter methods, show striking similarity to that observed by electron microscopy. The significance of ribosomal crystallography stems from the fundamental value of the better understanding of a basic life process, as well as from its potential applicative aspect. Thus, understanding the normal processes should shed light on abnormal and pathological deviations and provide the basic principles for the design of powerful and efficient therapeutic agents. These should benefit also from the illumination of the mode of binding of the antibiotics which block various steps of protein biosynthesis, and on the ribosomal interactions with RIPs (ribosomal inhibitors, mainly from plants) and with starvation- and heat shock proteins. Last, but not least, these studies are bound to contribute to the development of sophisticated crystallographic experimental methods.